In traditional plasma spraying a plasma flame is generated using a torch generally water cooled and having a tungsten cathode and a conical copper anode. Reactant which may be liquid, gaseous, solid or mixtures thereof, is entrained into the hot plasma flame by injection radially to the plasma jet. If the reactant is a powder it is generally carried by and driven into the plasma jet by a carrier gas.
The reactant is injected radially into the plasma flame either within the anode channel (nozzle) or a short distance from the nozzle.
With radial injection of powders the heating and dispersion of the injected reactant is strongly dependent on the trajectory of the reactant into the plasma flame jet. For powders these trajectories are determined by particle size, density, injection velocity and morphology and the range of trajectories is dependent on, among other variables, the size distribution of the powders being injection.
Axial injection of reactants has been used in thermal spray torches, see for example the supersonic velocity flame spray torch of Metco Diamond Jet, however, these spray torches are limited to reactants having low melting points (generally below about 1600.degree. C.) and have not been able to spray higher melting point materials.